Lateral variations of shear velocity structure in the western United States are studied using observations of Rayleigh- and Love-wave phase velocity dispersion. We measured Rayleigh-wave dispersion in the 6-60 sec period range on over 80 paths using both single- and two-station methods. Love-wave dispersion was also measured on about 50 paths. A pure-path regionalization method employing a block-province grid is used to determine lateral variations of dispersion in the Basin and Range and adjacent geologic regions. Structural inversions of the regionalized Rayleigh-wave dispersion curves indicate significant lateral variations in the crust and upper mantle structure of the Basin and Range and transition areas. A pronounced low-velocity zone in the upper mantle is found under the Eureka heat flow province in the central Basin and Range. The lithospheric thickness in this province is 45 km in contrast to the Great Basin model of Priestley and Brune (1978; ~65 km). Regionalization of Love-wave data, together with the Rayleigh-wave results, suggest the existence of polarization anisotropy in many areas of the Basin and Range. If verified, anisotropy must be accounted for in phase velocity inversions for the structure of the Basin and Range.